An actuating unit of this kind has already become known by the German patent application published without examination, No. 3,401,402. It is the special feature of this state-of-the-art actuating unit that the valve piston sealedly passes through a wall of the master brake cylinder and in the event of an actuation is advanced into the primary pressure chamber of the master cylinder. The purpose achieved by these provisions is that in the event of a failure of the boosting power, the operating element will act on a piston area whose active cross-sectional area is smaller than the piston area which undergoes the action of the boosting power.
Mounting of the prior-art actuating unit to the splash wall of the automotive vehicle is carried out by means of fixing bolts which are secured to the booster housing and which are inserted through openings in the splash wall and are provided with nuts.
The state-of-the-art arrangement in which the operating element, the vacuum brake power booster, and the master brake cylinder are in series with one another has the following disadvantages:
a) The reaction resulting from the hydraulic pressure which exists within the master brake cylinder is transmitted to the splash wall through the booster housing. As a result, deformations of the booster housing are caused which have a negative effect on the brake system.
b) Expansions are caused by elevated temperatures in the engine compartment and must be balanced by longer actuating travels of the operating element.
c) A relatively large overall axial length results which causes considerable problems as to mounting within the engine compartment.
d) Elevated transverse forces are created during actuation which are absorbed by an outward bending of the piston rod which actuates the valve piston and by the sealing ring guiding and sealing the control housing within the booster housing, which are transmitted to the master brake cylinder, so that they cause problems of guiding in the master cylinder.
e) The tolerances of the elements which participate in the power transmission between the operating element and the master cylinder are stacked.
f) Noises are caused by the aspiration of the air from the passenger compartment.
g) A reduction of the cross-sectional suction area of the air inlet opening in the control valve housing the presence of the piston rod.